This invention relates to polyurea or polyurea-polyurethane elastomers, more particularly to polyurea or polyurea-polyurethane elastomers which are prepared in part from low equivalent weight polyamines.
Polyurea and polyurea-polyurethane elastomers are commonly prepared using a low equivalent weight polyamine as a "chain extender" to increase the flexural modulus (stiffness) thereof. Flexural modulus generally increases with an increasing amount of the chain extender so that when a high modulus elastomer is desired, it is general practice to increase the amount of amine chain extender used in its preparation.
However, the use of large amounts of these amine chain extenders causes the elastomer to exhibit poor "green strength", i.e. the elastomer is quite brittle when removed from its mold. The lack of green strength renders the elastomer difficult or impossible to handle without distorting or cracking. For this reason, these amine chain extenders are largely restricted to use in preparing elastomers of low to moderate flexural modulus.
It has been attempted to overcome this problem of poor green strength by employing a high mold temperature. This has resulted in some improvement in green strength, but has the substantial disadvantage of causing polymer degradation, providing a polymer having poor surface qualities and of being relatively hazardous due to the need to handle a very hot mold.
Similarly, attempts to solve the green strength problem by employing an organometallic catalyst have been made. Unfortunately, the catalysts have been found to be unstable in amine-containing formulations, and tend to cause polymer degradation at high cure temperatures.
Although other low equivalent weight compounds such as polyols can be used to replace all or part of the amine chain extender, its use provides benefits such as excellent thermal properties (good heat sag and high heat distortion temperature). The replacement of the amine chain extender tends to adversely affect these properties. For this reason it is desirable to employ a high proportion thereof in preparing elastomers.
It would be desirable to provide a polyurea or polyurea-polyurethane elastomer derived from a relatively high proportion of an amine chain extender which also exhibits sufficient green strength to permit easy handling of the elastomer at demold.